The present invention relates to a method for the production of compacted graphite cast iron. The method is based on a test-method to determine the structural characteristics of a melt for casting compacted graphite cast iron and means to control the structure and properties of the material produced.
U.S. Pat. No. 4 667 725 describes a method for producing castings with a predetermined graphite structure. The method comprises taking a sample from a bath of molten iron and then allowing the sample to solidify over a period of from 0.5 to 10 minutes. During the solidification period, recordings are made of the temperature changes that take place in the centre of the sample volume and near the inner wall of the sample vessel. The sample vessel should substantially be in thermal equilibrium at a temperature above the crystailization temperature of the bath and be allowed to solidify fully over a period of from 0.5 to 10 minutes. The temperature-time sequence is is measured by two temperature responsive means, of which one is placed in the centre of the sample quantity and the other in the molten material at a location close to the wall of the vessel. The degree of dispersion of the graphite phase in relation to known reference values for the same sampling and testing process with respect to finished castings is assayed with the aid of the temperature measured during the first nucleation events of the eutectic reaction measured at said vessel wall as the minimum undercooling temperature at the vessel wall, the recalescence of the vessel wall, the positive difference between the temperature that prevails at the vessel wall and in the centre of the sample and the derivative of the temperature decrease at said vessel wall during the time for constant eutectic growth temperature in the centre of the sample quantity, alternatively the highest negative values of the temperature difference between the eutectic maximum growth temperature and the eutectic temperature. If the bath has an insufficiency of crystallization nuclei, a graphite nucleating agent is introduced thereinto and conversely when the crystallization nucleants are present in excess the degree of dispersion is lowered by holding the bath for a length of time sufficient to reduce the amount of nuclei in the bath prior to casting, by assessing the morphology of the graphite precipitation in relation to corresponding data obtained with the same sampling and testing technique as that applied with cast iron of known structure, with the aid of supercooling that takes place in the centre of the molten material, the recalescence in the centre of the sample vessel and the maximum growth temperature, and by correcting the quantity of structure-modifying agent in response thereto, so that graphite precipitation takes place in a predetermined form upon solidification of the molten cast iron subsequent to casting. The obtained values are used to determine the amount of modifying agent and the presence of graphite nucleating agent in the melt concerned and for the equipment used. The production equipment used has to be calibrated and possibly necessary additions of modifying element and nucleating agent have to be made in relation to the calibration of the best equipment.
The above-mentioned method permits the skilled person to predict how the graphite precipitation will take place in the melt concerned and also how to control the composition of the melt to obtain desired results. The test method gives results which cannot be obtained with other analysis methods. Although a chemical test will, for instance, disclose the total amount of magnesium present, it will provide no information as to how much magnesium there is in solution and thus active as a modifying agent. This latter information is of great importance since the amount of magnesium in solution in an iron melt will change relatively quickly due to physical and chemical reactions that take place within the melt and due to contact with the surroundings and consequently although a conventional chemical analysis may have given the correct results in respect of the melt concerned at the time of making the analysis may well have changed its status at the time of receiving the analysis results to such an extent that this result can no longer be used to control the solidification process, when casting cast iron objects.
The amount of dissolved elemental magnesium is essential for controlling the graphite precipitation- In addition to magnesium or instead of magnesium the structure modifying agent can contain cerium and other rare earth metals. The modifying agent (and the nucleating agent) will fade with time and there are characteristic rates for this fading, contingent on the process and equipment used. A good control method will enable an exact determination to be made of the amount of modifying and inoculating agents present and will also enable a calculation to be made as to how much of these agents are needed to obtain acceptable results for a casting process during the following say 10 to minutes. This has not, however, been possible hitherto. The method described earlier can only inform us that the melt at the sample extraction moment will solidify with specific graphite crystal form.
The undercooling in a melt where flaky graphite crystals develop is relatively small (&lt;5.degree. C.) and the minimum represents the situation where a certain number of graphite crystals (together with the austenite phase) have attained a growth rate at which the latent heat evolved balances the heat extracted from the system. After this point, the liquid melt is actually heating up to a new balance point representing the "steady state growth condition", which in the case of a well nucleated liquid with A-graphite is close to the equilibrium temperature T.sub.E say 1.degree.-2.degree. C. below the equilibrium temperature of the eutectic reaction in normal Fe-C-Si-alloys. In the present document, the equilibrium temperature is set at 1155.degree. C. This means that the instruments have been calibrated to a T.sub.E =1155.degree. C. and temperature differences calculated in relation therewith. Thermodynamic calculations in the literature give other, somewhat higher vlaues, but for practical reasons T.sub.E is in this connection set at 1155.degree. C.
If modifying agents such as magnesium and rare earth metals are added and dissolved in a liquid cast iron melt, the growth in specific crystallographic directions is restricted and the morphology changed from flaky via compacted to nodular graphite crystals with increasing amounts. From type IV to I in the graphite classification scale.
If there are sufficient graphite precipitation nuclei present and a proper amount of modifying elements is present, the cast iron will solidify as compact graphite cast iron. In this case, the undercooling is much higher than is observed for gray iron before the rare of growth of compacted graphite crystals generates enough heat to counteract and heat-extraction from the system.
The reheating (recalescence) takes a longer time, due to growth restrictions, and the steady state growth temperature will stay from 5.degree. to 10.degree. C. below the equilibrium liquidus temperature, T.sub.E.
The relationship between the growth habit of these two graphite morphologies and the thermal analysis results and curves have long been known.
The successive change in morphology from flaky to compacted graphite as a result of increasing additions of modifying elements is, however, of interest because of its great importance in enabling thermal analysis to be used as a process control instrument.
This change is by no means a linear function of the concentration of modifying additives, for the following reasons:
Here, magnesium is referred as an example of the modifying agent used and when adding a modifying agent, the magnesium will react with any sulphur and oxygen that may be present while forming MgS and MgO. The remainder of the magnesium will dissolve in the cast iron melt and is defined as Mg.sub.(L).
In a set of experiments, it has been found that a Mg.sub.(L) -level of 0,008% Mg will give a fully compacted graphite structure and an Mg.sub.(L) -level below 0.006% Mg will give a fully flaky structure in a slightly hypoeutectic cast iron (the carbon equivalent CE of 4.0-4.2). A level of dissolved magnesium below 0.006% will not be sufficient to prevent the formation of flaky graphite. This formation will release latent heat at such a high rate as to reduce the degree of undercooling, and the growth of compacted graphite crystals is never triggered. At this lower degree of undercooling, some crystals may, however, be influenced by the modifying agent to such an extent that they develop in a somewhat unspecified modified form.
Compacted graphite crystals are formed in the range between 0,008-0,016% dissolved magnesium. No flaky crystals are formed in this range, but towards the higher end of this range a certain formation of nodular crystals can be observed.
The absolute values of the content of dissolved elemental magnesium may vary from one foundry to another and with constituents in the base melt, but in one actual case the following figures were obtained, which may serve as an example:
0-0.008% Mg, flaky graphite PA0 0.008-0.016% Mg, compacted graphite PA0 0,016-0.030% Mg, mix of compacted and nodular graphite PA0 0,030-0.035% Mg, 80-100% nodular graphite PA0 &gt;0.035% Mg, fully nodular graphite (Mg in excess)
From one practical application, it was found that for the process and equipment used, the fading of magnesium was about 0.001% Mg every five minutes. During a casting period of 15 minutes, the content of dissolved elemental magnesium thus decreased with 0.003%. If the original percentage was 0.010% Mg, one would obtain after 15 minutes a content of 0.007% and a large portion of flaky graphite will thus be formed by the end of the casting period, while a sample that originally contains say 0.012% Mg will form only compacted graphite crystals over the whole casting period. If the melt had been tested according to U.S. Pat. No. 4,667,725, it would have been found in both cases, quite correctly, that both melts would solidify as compacted graphite cast iron. The method does not permit a discrimination to be made between magnesium contents of 0.010% and 0.012%. As graphite flakes have to be completely avoided, it has so far been necessary to use a certain excess of magnesium in order to secure a desired result, with the risk of obtaining a certain amount of nodular graphite crystals in the material being produced.
The decrease of the content of dissolved elementary magnesium below about 0.008% will result in a very rapid increase of the amount of flaky graphite due to the fact that an insufficient amount of magnesium permits the growth of flakes of graphite and that a decrease of the magnesium content below the borderline will result in a drastic transformation of the solidification structure with regard to the graphite precipitation. This will be obvious from the Figure, where the amount of compacted graphite and flaky graphite is shown along the ordinate and the percentage of dissolved magnesium along the abscissa. The curve shows the drastic change that occurs, when the magnesium content drops below 0.008% magnesium. Other modifying agents have similar threshhold concentrations.
Once the growth habit of the graphite crystals if fully compacted, the test will result in mutually identical signals from the analysis equipment within the range concerned, i.e. between 0,008-0.16% Mg.sub.(L). A chemical analysis is not useful since there is no fast method known of discriminating between total and dissolved magnesium quantities.